Problem: Solve for $x$ and $y$ using substitution. ${-3x-4y = 4}$ ${x = -4y+12}$
Answer: Since $x$ has already been solved for, substitute $-4y+12$ for $x$ in the first equation. ${-3}{(-4y+12)}{- 4y = 4}$ Simplify and solve for $y$ $12y-36 - 4y = 4$ $8y-36 = 4$ $8y-36{+36} = 4{+36}$ $8y = 40$ $\dfrac{8y}{{8}} = \dfrac{40}{{8}}$ ${y = 5}$ Now that you know ${y = 5}$ , plug it back into $\thinspace {x = -4y+12}\thinspace$ to find $x$ ${x = -4}{(5)}{ + 12}$ $x = -20 + 12$ ${x = -8}$ You can also plug ${y = 5}$ into $\thinspace {-3x-4y = 4}\thinspace$ and get the same answer for $x$ : ${-3x - 4}{(5)}{= 4}$ ${x = -8}$